In various processes such as paper making, printing and coating, a lengthwise moving web is, at some point in its Path, brought into partial wrapping engagement around a rotating roller so that the web can have intimate contact with the cylindrical surface of the roller for heat transfer or for some other purpose. A problem that has heretofore persisted in connection with such processes is that there is a tendency for a film of air to intrude between the web and the cylindrical surface of the roller, preventing the desired contact between them.
It is known that a relatively thin "boundry layer" of air is picked up by the moving surfaces of the web and the roller and that some of this air becomes trapped in the wedge-shaped space where the web approaches the roller surface. Unless the web is under a relatively high lengthwise tension, or is moving lengthwise at a relative low speed, the trapped air enters between the roller and the portion of the web that curves around it, forming a film between the roller and all of that portion of the web that is wrapped around it.
If web speed is low enough and the web is under sufficient lengthwise tension, the trapped air in the above-mentioned wedge-shaped space is repelled by the pressure of the web pushing onto the cylindrical surface of the roller. The pressure p exerted by the web in pushing onto the roller surface, in pounds per square inch (psi) and assuming 180.degree. wrap on the chill roll, is given by: EQU p=t/r,
where t is web tension in pounds per lineal inch (pli), and r is cylinder radius in inches.
Thus, if a paper or plastic web is under a typical tension of 2 psi and is running around a 12-inch diameter roller, the pressure that pushes the web towards the roller surface is 1/3 psi. If the speed of the web and cylinder is very low (e.g., less than 100 fpm) a 1/3 psi web pressure is high enough to almost completely repel the air in the wedge-shaped space from entry between the roller and the portion of the web that curves around it, and the web will make reasonably good contact with the roller surface. Of course, perfect smoothness of the web and roller surfaces is unattainable in practice, and some air will be present between those surfaces in the void spaces defined by surface irregularities, but there will be substantial surface-to-surface contact in contrast to the substantially total separation between the surfaces that exists when a film of air is present.
It will be evident that where a web is to be heated or cooled by a roller around which it is partially wrapped, an insulating film of air between the web and the roller will materially reduce the efficiency of heat transfer. If a freshly imprinted or coated web is passed through an oven and is then brought to a chill roll to be cooled, an air film that intervenes between the web and the chill roll prevents cooling of the web to the temperature it is intended to have upon moving away from the chill roll, and troubles may be encountered in subsequent stages of processing of the web.
Furthermore, the air film may allow solvent to condense on the chill roll surface, forming rather thick layers of ribbons of condensate that the web intermittently reabsorbs in sufficient amounts to resoften the ink. Heatset inks require residual solvent levels of about 10% to 15% in the final product to maintain product quality. Once heated, these solvents continue to evaporate as long as the web temperature is above about 170.degree. F. As web lift off begins, solvent starts to accumulate on the chill roll. Actual accumulation amounts are dependent on coverage, tension, speed and dryer operating parameters.
In web winding and rewinding operations, wherein a substantial length of web is wound onto itself to form a continuous roll, air trapped between the oncoming web and the already-wound part of the roll can form a film between successively wound layers, resulting in a roll that has an excessive diameter, is too loosely wound, and may create problems during subsequent handling or use, as by telescoping when tilted.
Again, where an idler roll is to be driven by means of a moving web, a thin film of air between the web and the roll reduces the friction force needed for driving the roll, and serious slippage between them may result.
The development of an air film between a web and a roller around which it has partial wrapping engagement can sometimes be avoided by mounting a pressure roller in juxtaposition to the roller to be contacted by the web, whereby the web is literally squeezed into contact with that roller. However, there are many situations in which this expedient cannot be used because the web surface that faces away from the roller to be contacted cannot tolerate engagement by a solid object.
U.S. Pat. No. 3,452,447, issued to T. A. Gardner in 1969, points out that holding a web tightly to a drum such as the steam cylinder of a dryer "has long presented problems" due to entrained air trapped between the web and the drum, "thereby greatly reducing the transfer of heat". The patent proposes to mount an air bar to blow air against the web from the side of it that is opposite the drum, the air bar being positioned along the line at which the web is tangent to the drum. The patent recognizes that blowing air directly towards the web in an effort to force it into contact with the drum would normally be ineffectual because the air jet or jets, after impacting the web, would be deflected or redirected by it into flow along its surface that would produce a lift effect; and "the lift effect of the redirected jets is sufficiently great so that it tends to nullify the pressure exerted by the jets". Instead, Gardner's air bar has a pair of outlets which are spaced apart by a small distance in the direction of movement of the web and from which air jets issue towards the web at opposite substantially oblique angles to its surface such that they converge towards one another. The convergent air jets are said to produce a pressure zone between the air bar and the web, in the region between the outlets from which they are emitted, and the patent states that "the pressure exerted over the relatively large area of the pressure zone [is] so much greater than the lift effect of the redirected jets that the latter ceases to be of any consequence".
The expedient disclosed by Gardner may be of value where web tension is rather high--as expressly contemplated by the patent--and with moderate web speeds, but it is doubtful that it would be effective with relatively high web speeds and small or moderate tensions. In all cases it would require a substantially high rate of air flow to be effective and would therefore consume a substantial amount of energy in its normal operation.
U.S. Pat. No. 4,369,584, assigned to the assignee of the instant invention, discloses the use of a high velocity air jet to force a moving web into contact with a rotating roller, such as a chill roll. Although such an approach has been successful, the jet demands a substantial energy requirement to generate the high pressure air.
U.S. Pat. No. 4,462,169 also assigned to the assignee of the instant invention, discloses a chill nip which depends upon the use of an interference fit. Thus two cooperating rolls form an adjustable nip clearance maintained at about 0.001 inches less than the thickness of the web. However, the resulting physical compression of the web can damage not only the printed surface, but the web itself.